The present invention relates to the field of manipulating light images in the visible and infrared regions.
Frequently, a machine vision system is required to measure the orientation, distance and three-dimensional coordinates of an object in its field of view. An attractive means to accomplish this is by the use of structured illumination, usually in the form of a narrow laser generated stripe or double stripe crosshair. When using this method, the optical path to the camera typically includes some type of narrow bandpass filter that allows the laser light wavelength such as 780 nm for a laser-diode to pass through, while blocking light of other wavelengths. This simplifies the image analysis problem for the light stripe. An example of such a system is a robotic seam tracker.
Conversely, many machine vision applications such as inspection systems, require lighting that contains all visible wavelengths (white light) as an illumination source in order to maximize contrast. These systems typically require the lighting to be over the entire field-of-view, as opposed to the aforesaid narrow stripe.
A problem arises when a machine vision system is required to perform both the 3-D measurement and the inspection functions, or when the centroid of the object is not apparent from the projection of the laser, i.e. when the object is relatively planar. An example of a system with such a requirement is the robotic derivating cell for the U.S. Air Force. This system locates and removes rivets and other fasteners from aircraft components such as airlerons and wingflaps. The first step in the removal process is for the machine vision system to find the 3-D coordinates of the fastener and identify the type of fastener. To calculate the 3-D coordinates, a laser crosshair image is used to first calculate the plane of the component surface in 3-D space in which the component lies. Floodlight illumination is then used to locate the center of the rivet within this plane, and also to identify the rivet type.
It is critical in machine vision applications that the lighting be optimized for each function. Unfortunately, the two functions stated above have conflicting goals with respect to the lighting and optics requirements. The 3-D location function wants to see only the laser crosshair image while the fastener location and fastener identification function requires the entire field of view evenly illuminated.
Prior to the present invention, the aforesaid problem might of been addressed through the use of complicated electro-mechanical shuttering or mirror arrangements. However, the two types of images would still not be simultaneously available. Also these arrangements would increase the cost and complexity of the machines while reducing the ruggedness and reliability of the system. Although compromise lighting systems are an alternative approach, they generally tend to degrade the performance of each function.